The present invention refers to improvements in devices used for copying images by sequential sweeping, and it has as its main object a new structure of the optical system which is a part of those devices.
Conventionally, devices of the above-mentioned type used for producing stencil reproductions of indicia provided on an original document include a rotating drum where there is placed on one side of the original to be copied, and in an adjacent position, the material which is to receive the image. Laterally to said cylinder or drum there is placed a movable structure which supports the optical system which records and transforms into electrical levels the various light and dark tones of a particular zone of the original.
On the same carrying system and forming a rigid unit, there is placed a wire or needle (stylus) which rests with some constant pressure on the image-receiving material. When the optical system registers a dark zone, there is applied to said wire a train of high frequency impulses, which perforates and darkens the sensitive material. The cylinder is made to rotate at a uniform speed, usually between 200 and 800 rpm, while the carrying system for the optical system and the wire moves slowly in a rectilinear direction parallel to the cylinder or drum. For each rotation of the cylinder, the carrying system moves forward one fraction of one millimeter, sweeping in that way, in a few minutes, all of the area of interest.
The equipment provided up until now has a limited field of use, because of the quality of the images reproduced. The most exploited field is the making of stencils for mimeographs.
The receiving sheet is a thin sheet of plastic material or of paper, which is perforated by dielectric disruption. Through those perforations there passes the ink to execute the printing.
The above system is also used to make matrices for offset, in which the electric arc perforates a thin hydrophilic layer, exposing a hydrophobic (water-repellent) layer which receives the ink.
There can be obtained black and white copies on sensitive material which is made up of a sheet of paper with a thin layer of plastic provided with a white pigment, which becomes perforated exposing under it a layer of plastic with conductive carbon. In this manner there are obtained copies showing great contrast, but that system is not competitive with more rapid systems such as Xerox or Electrofax.
The improvements of the optical system of the present invention make it possible to improve the quality of the stencil making system to such an extent that, not only does it widen the possibilities of the stencil for use with mimeograph machines and for offset matrices to fields having greater requirements, but make it competitive for the reproduction of photographic copies in place of silver bromide paper. Advantages realized with the present system are lower cost of copies, copies of relatively high quality and low cost of equipment.
The structure of optical systems known in the art includes at least one lamp the filament of which is concentrated through a pair of lenses onto the original document mounted on the cylinder. The illuminated diaphragm, is focused by means of another lens on a diaphgram, which has a small perforation which determines the size of the area being explored. The light which passes therethrough excites a photomultiplier. For the light levels available, the most common practice is to use a photomultiplying tube of a well known type. For proper functioning of that optical complex or arrangement, lamps must be used with filaments having a controlled position, and the alignment of the system must be accurately set to make the optical axes of the three lenses coincidental. To maintain the alignment, there is required a mounting system which supports the latter and does not transmit the heat of the lamps to the original as there would be produced thermal currents in its parameters. All of this results in a high cost of construction, and drawbacks in the use.